Editing IPCC:AR6/WGII/Chapter-2 (section)

=== 2.5.4 Key Risks to Terrestrial and Freshwater Ecosystems from Climate Change ===

<div id="h2-15-siblings" class="h2-siblings"></div>

Among numerous risks to terrestrial and freshwater ecosystems from climate change, this chapter identified five phenomena as the most fundamental risks of climate change to ecosystem integrity and the ecosystem services that support human well-being that are also quantified sufficiently to estimate risk thresholds with at least ''medium'' confidence : Biodiversity loss (global losses of species from ecosystems), ecosystem structure change, increased tree mortality, increased wildfire, and ecosystem carbon losses and (Table 2.5, Table SM2.5; Figure 2.11). These key risks form part of the overall assessment of key risks in Chapter 16. The AR5 chapter on terrestrial ecosystems ( [[#Settele--2014|Settele et al., 2014]] ) had also identified three of these key risks—species extinctions, tree mortality and ecosystem carbon losses—and a fourth—invasion by non-native species. This chapter assesses, in multiple sections, the impacts of climate change on invasive species with respect to different processes or systems (e.g., in [[#2.4.2.3.3|Section 2.4.2.3.3]] ), and includes this aspect here in a new broader key risk of ecosystem structure change. The AR5 included wildfire as a mechanism of the key risk of ecosystem carbon loss. Based on additional research and field experience with major wildfires since then, this chapter sets wildfire apart as a specific key risk to ecosystem integrity and human well-being. These different measures of risk are interconnected, but approach the assessment of the risks to terrestrial and freshwater ecosytems from different angles, using complementary metrics.

Species are the fundamental unit of ecosystems. As species become rare, their roles in the functioning of the ecosystem diminishes and disappears altogether if they become locally extinct ( ''high confidence'' ) ( [[#Isbell--2015|Isbell et al., 2015]] ; [[#Chen--2018b|Chen et al., 2018b]] ; [[#van%20der%20Plas--2019|van der Plas, 2019]] ; [[#Wang--2021b|Wang et al., 2021b]] ). Loss of species and functional groups reduces the ability of an ecosystem to provide services, and lowers its resilience to climate change ( ''high confidence'' ) ( [[#2.6.7|Section 2.6.7]] ) ( [[#Elmqvist--2003|Elmqvist et al., 2003]] ; [[#Cadotte--2011|Cadotte et al., 2011]] ; [[#Harrison--2014|Harrison et al., 2014]] ; [[#Carlucci--2020|Carlucci et al., 2020]] ). For example, among crop systems, a key factor to succesful pollination is the phylogenetic diversity of bee species available, more than total abundances ( [[#Drossart--2020|Drossart and Gérard, 2020]] ). Because many species have obligate interactions with, or are resources for, other species (e.g., predators and their prey, insects and their host plants, plants and their mycorrhizae symbionts), the loss of one species affects the risk to another species, and, ultimately, ecosystem functioning ( [[#Mahoney--2017|Mahoney and Bishop, 2017]] )

Global rates of species extinction are accelerating dramatically ( [[#Barnosky--2011|Barnosky et al., 2011]] ), with approximately 10% of species having been driven extinct by humans since the late Pleistocene, principally by overexploitation and habitat destruction, a rate estimated to be 1000 times higher than pre-Anthropocene (natural) background extinction rates ( [[#De%20Vos--2015|De Vos et al., 2015]] ). Therefore, this level—10%—of species becoming “endangered” (sensu IUCN),and therefore at ''high'' risk of extinction, due to the loss of suitable climate space (Figure 2.8b), is used here as a threshold, moving the risk to biodiversity from ''moderate'' to ''high'' , and twice that (20%) as the threshold from ''high'' to ''very high'' .

Key risks assessed here are interconnected. Extinction of species is an irreversible impact of climate change and has negative consequences on ecosystem integrity and functioning, and the risks increase steeply with even small rises in global temperature ( [[#2.5.1.3|Section 2.5.1.3]] , Figure 2.6, Figure 2.7, Figure 2.8). Continued climate change substantially increases the risk of carbon losses due to wildfires, tree mortality from drought and insect pest outbreaks, peatland drying, permafrost thaw and changes in the structure of ecosystems; these could exacerbate self-reinforcing feedbacks between emissions from high-carbon ecosystems and increasing global temperatures ( ''medium confidence'' ). Thawing of Arctic permafrost alone could release 11–200 GtC ( ''medium confidence'' ). Complex interactions of climate changes, LULCC, carbon dioxide fluxes and vegetation changes will regulate the future carbon balance of the biosphere, processes incompletely represented in ESMs. The exact timing and magnitude of climate–biosphere feedbacks and the potential tipping points of carbon loss are characterised by broad ranges of the estimates, but studies indicate that increased ecosystem carbon losses could cause extreme future temperature increases ( ''medium confidence'' ). (Sections 2.5.2.7, 2.5.2.8, 2.5.2.9, 2.5.3.2, 2.5.3.3, 2.5.3.4, 2.5.3.5, Figure 2.10, Figure 2.11, Table 2.4, Table 2.5, Table SM2.2, Table SM2.5)

'''Table 2.5 |''' Key risks to terrestrial and freshwater ecosystems from climate change. This IPCC chapter assesses these as the most fundamental risks of climate change to ecosystem integrity and the ecosystem services that support human well-being. Climate factors include the primary variables governing the risk. Non-climate factors include other phenomena that can dominate or contribute to the risk. Detection and attribution comprise cases of observed changes attributed predominantly, or in part, to climate change, with some cases being attributed to anthropogenic climate change (Sections 2.4.2, 2.4.3, 2.4.4, 2.4.5, Table 2.2, Table 2.3, Table SM2.1). Adaptation includes options to address the risk (Section 2.6). Risk transitions (defined in Figure 2.11) indicate an approximate GSAT increase, relative to the pre-industrial period (1850–1900), to move from one level of risk to the other as well as assessed confidence. Table SM2.5 provides details of the temperature levels for risk transitions. Both tables provides details for the key risk burning embers diagram (Figure 2.11).

{| class="wikitable"
|-
| colspan="5"| '''Global biodiversity loss:''' Increasing numbers of plant and animal species at ''high'' extinction risk (species becoming endangered with projected loss of &gt;50% of range). The transition from non-detectable risk to moderate risk was based on the observed documentation of hundreds of local population extinctions, major declines in many sub-species and two to 92 global species extinctions that are attributable to climate change (with ''medium confidence'' or higher). The transition from ''moderate risk'' to ''high risk'' of biodiversity loss is centred around 1.5°C, based on a few taxa that are known from their basic biology and habitat requirements to be at ''high'' risk of extinction (endangered) at 1.5°C, and on the increasing number of taxa that are projected to have a ''high'' extinction risk affecting &gt;10% of the species in that taxa (1000 times the natural background rates of extinction). The transition to ''very high risk'' of biodiversity loss comes from the increasing number of taxa projected to have &gt;20% of species at a ''high'' risk of extinction. In the worst-case scenario (10th percentile of the models), some of the taxa show &gt;50% of the species at a ''high'' risk of extinction. These assessments are also weighted by role the species in the taxa play in performing ecosystem services (both to the ecosystems and to humans, e.g., pollinators, detritivores). There is ''high confidence'' for the moderate risk threshold because it is based on observed trends attributed to climate change. There is ''medium confidence'' for future projections since, for the purpose of developing this burning embers diagram, these risk thresholds are based on one large study (covering &gt;119,000 species) for which there were multiple warming scenarios considered, and primarily on the loss of suitable climate. Based on Sections 2.4.2, 2.5.1, 2.6.1, 2.6.6, Table 2.3, Figure 2.6, Table SM2.1 and Table SM2.2.

|-
| '''Climate factors'''

| '''Non-climate factors'''

| '''Detection and attribution'''

| '''Adaptation'''

| '''Risk transitions'''

'''(''' '''''confidence''''' ''')'''

|-
| Shifts in geographic placements of climate space; loss of climate space globally; emergence of non-analogue climates, increases in extreme climate events

| LUC, habitat degradation (e.g. from pollution, fertilisation, and invasive species)

| Already observed: many cases of population extinctions; 2 to 92 cases of species extinctions (2.4.2.2, 2.4.2.7.1);

species have tracked their climate niches raising confidence in SDM projections (2.4.2.1, 2.4.2.3, 2.4.2.5)

| Habitat restoration, habitat creation, increased connectivity of habitats and protected areas, increase in protected areas, assisted colonisation

| 0.8°C undetectable risk to moderate risk ( ''high confidence'' )

1.58°C moderate risk to high risk ( ''medium confidence'' )

2.07°C high risk to very high risk ( ''medium confidence'' )

|-
| colspan="5"|
|-
| colspan="5"| '''Ecosystem structure change:''' increasing risk of large-scale changes in ecosystem structure. Ecosystem structural change with most information derived for tropical forests, boreal forests, savannas and tundra for both observations and future projections. The transition from ''non-detectable risk'' to ''moderate risk'' is based on detected changes attributable to climate change or to interactions between changing disturbance regime, climate and rising CO 2 . These changes have already been observed at 0.5°C above pre-industrial levels, with shifts initially detected in boreal forests, tundra and tropical grassy ecosystems. The transition from ''moderate risk'' to ''high risk'' is centred around 1.5°C, based on widespread global observations (at a current GSAT of 1.09°C above pre-industrial levels) that agree with projected future impacts with at least 10% area of key ecosystems being affected (Box 2.1). Overall, there is ''medium confidence'' in projections. This is based on existing observations and some projections that have a ''high confidence'' of risk for several ecosystems, but data and projections are not available for all biomes, thus lowering overall confidence to ''medium confidence'' . The transition from ''high risk'' to ''very high risk'' occurs when &gt;50% of multiple ecosystems are projected to experience shifts in structure. (Sections 2.4.2.3, 2.4.3, 2.4.5, 2.5.2, Box 2.1, Figure Box 2.1.1, Table Box 2.1.1, Table SM2.2, Table SM2.3, Table SM2.4, Table SM2.5)

|-
| '''Climate factors'''

| '''Non-climate factors'''

| '''Detection and attribution'''

| '''Adaptation'''

| '''Risk transitions'''

'''(''' '''''confidence''''' ''')'''

|-
| Increases in average and extreme temperatures, changes in precipitation volume and timing, increased atmospheric CO 2

| LUC, livestock grazing, deforestation, fire suppression, loss of native herbivores, food, fiber and wood production

| Individual species range shifts, biome shifts

| Conservation of potential refugia, habitat restoration, increasing connectivity of habitats and protected areas, increase in protected areas, changes in grazing and fire management

| 0.5°C undetectable rsik to moderate risk ( ''high confidence'' )

1.5°C moderate risk to high risk ( ''medium confidence'' )

2.5°C high risk to very high risk ( ''medium confidence'' )

|-
| colspan="5"|
|-
| colspan="5"| '''Tree mortality:''' tree mortality that exceeds natural levels degrades habitat for plant and animal species, increases carbon emissions and reduces water supplies for people. Anthropogenic climate change caused three cases of drought-induced tree mortality in the period 1945–2007 in western North America, the African Sahel and north Africa in temperate and tropical ecosystems. Increased pest infestations and wildfires due to climate change also caused much of the recent tree mortality in North America. These changes occurred at GMST increases of 0.3°C–0.9°C above those in the pre-industrial period. Models project increasingly extensive drought-induced tree mortality at continued temperature increases of 1°C–2°C. Models project risks of mortality of up to half the forest area in different biomes at temperature increases of 2.5°C–4.5°C. In Amazon rainforests, insufficient plant moisture reserves during drought increase the risk of tree mortality, and, combined with increased fire from climate change and deforestation, the risk of a tipping point of massive forest dieback and a biome shift to grassland. (Sections 2.4.4.3, 2.5.2.6, 2.5.3.3, 2.5.3.5)

|-
| '''Climate factors'''

| '''Non-climate factors'''

| '''Detection and attribution'''

| '''Adaptation'''

| '''Risk transitions'''

'''(''' '''''confidence''''' ''')'''

|-
| Increase in temperature, decrease in precipitation, increase in aridity, increase in the frequency and severity of drought

| Deforestation, LUC

| Tree mortality up to 20% in three regions in Africa and North America

| Reduce deforestation, reduce habitat fragmentation, encourage natural regeneration, restore fragmented habitats

| 0.6°C undetectable risk to moderate risk ( ''high confidence'' )

1.5°C moderate risk to high risk ( ''medium confidence'' )

3.5°C high risk to very high risk ( ''medium confidence'' )

|-
| colspan="5"|
|-
| colspan="5"| '''Wildfire:''' increasing risk of wildfire that exceeds natural levels, damaging ecosystems, increasing human diseases and deaths and increasing carbon emissions. Field evidence shows that anthropogenic climate change has increased the area burned by wildfire above natural levels across western North America in the period 1984–2017, increasing burned area up to 11 times in one extreme year and doubling burned area over natural levels in a 32-year period. Burned area has increased in the Amazon, the Arctic, Australia and parts of Africa and Asia, consistent with but not formally attributed to anthropogenic climate change. These changes have occurred at GMST increases of 0.6°C–0.9°C. Empirical and dynamic global vegetation models project increases in burned area and fire frequency above natural levels on all continents under continued climate change, the emergence of an anthropogenic signal from natural variation in fire weather for a third of the global area and increases of burned area in regions where fire was previously rare or absent, particularly the Arctic tundra and Amazon rainforest, at global temperature increases of 1.5°C–2.5°C. Models project up to a doubling of burned area globally and wildfire-induced conversion of up to half the area of the Amazon rainforest to grassland at temperature increases of 3°C–4.5°C. (Sections 2.4.4.2, 2.5.3.2)

|-
| '''Climate factors'''

| '''Non-climate factors'''

| '''Detection and attribution'''

| '''Adaptation'''

| '''Risk transitions'''

'''(''' '''''confidence''''' ''')'''

|-
| Increase in the magnitude and duration of high temperatures, decrease in precipitation, decrease in relative humidity

| Deforestation, agricultural burning, peatland burning

| Increased burned area in western North America above natural levels

| Reduce deforestation, reduce the use of fire in tropical forests, use prescribed burning and allow naturally ignited fires to burn in targeted areas to reduce fuel loads, encourage settlement in non-fire-prone areas

| 0.75°C undetectable risk to moderate risk ( ''high confidence'' )

2.0°C moderate risk to high risk ( ''medium confidence'' )

4.0°C high risk to very high risk ( ''medium confidence'' )

|-
| colspan="5"|
|-
| colspan="5"| '''Ecosystem carbon loss''' : increasing risk of ecosystem carbon losses that could substantially raise the atmospheric carbon dioxide level. Measurements have detected emissions of carbon from boreal, temperate and tropical ecosystems in places where increases in wildfire and tree mortality have been attributed to anthropogenic climate change, at GMST increases of 0.6°C–0.9°C above the pre-industrial period. Many factors govern the carbon balance of ecosystems, so changes have not been attributed to climate change. Tropical forests and Arctic permafrost contain the highest ecosystem stocks of above- and below-ground carbon, respectively. Due to deforestation and forest degradation, primary tropical forests currently emit more carbon to the atmosphere than they remove. Wildfires in the Arctic are contributing to permafrost thaw and soil carbon release. An emissions scenario of 2°C increase could thaw ~15% of permafrost area and emit 20–100 GtC by 2100. Under emissions scenarios of 4°C global temperature increase, models project possible tipping points of conversion of half the Amazon rainforest to grassland and thawing of Arctic permafrost that could release 11–200 GtC which could substantially exacerbate climate change (Sections 2.4.3, 2.4.4.3, 2.4.4.4, 2.5.2.7–10, 2.5.3.2–5, Figure 2.9, Figure 2.10, Figure 2.11, Table 2.4, Table 2.5, Table SM2.2, Table SM2.3, Table SM2.5).

|-
| '''Climate factors'''

| '''Non-climate factors'''

| '''Detection and attribution'''

| '''Adaptation'''

| '''Risk transitions'''

'''(''' '''''confidence''''' ''')'''

|-
| Increase in temperature, increase in aridity, increase in the frequency and severity of drought

| Deforestation, road and infrastructure expansion, agricultural expansion

| Losses of carbon detected in boreal, temperate and tropical ecosystems due to wildfire and tree mortality, not formally attributed to climate change

| Reduce deforestation, especially in tropical forests, reduce road and infrastructure expansion, especially in the Arctic, reduce the use of fire to clear agricultural land, increase protected areas

| 0.75°C undetectable risk to moderate risk ( ''medium confidence'' )

2°C: moderate risk to high risk ( ''medium confidence'' )

4°C high risk to very high risk ( ''low confidence'' )

|}

<div id="_idContainer053" class="Figure"></div>

[[File:9c3a25c8d5a4134bf5d1c11a873cfe74 IPCC_AR6_WGII_Figure_2_011.png]]

'''Figure 2.11 | Key risks to terrestrial and freshwater ecosystems from climate change.''' This IPCC chapter assesses these as fundamental risks of climate change to ecosystem integrity and the ecosystem services that support human well-being, based on observed impacts and future risks of: (far-left) “biodiversity loss” refers to losses of animal and plant species from different ecosystems globally, with resulting declines in ecosystem integrity, functioning and resilience ( [[#2.4.2.1|Section 2.4.2.1]] , 2.4.2.2, 2.5.1.3.3); (middle-left) “structure change” refers to major changes occurring in ecosystem structure (Sections 2.4.3, Box 2.1, 2.5.2, Figure 2.9, Figure Box 2.1.1, Table Box 2.1.1, Table SM2.5); (middle) “tree mortality” refers to tree mortality exceeding natural levels (2.4.4.3, 2.5.3.3); (middle- right) “wildfire increase” refers to wildfire exceeding natural levels ( [[#2.4.4.2|Section 2.4.4.2]] , 2.5.3.2); (far-right) “carbon loss” refers to ecosystem carbon losses that could occur abruptly and substantially raise atmospheric carbon dioxide (Sections 2.4.3.6–2.4.3.9, 2.4.4.4, 2.5.2.6–2.5.2.10, 2.5.3.4, 2.5.3.5). This burning embers diagram shows impacts and risks in relation to changes in GSAT, relative to the pre-industrial period (1850–1900). Risk levels reflect current levels of adaptation and do not include more interventions that could lower risk. The compound effects of climate change, combined with deforestation, agricultural expansion and urbanisation as well as air, water and soil pollution and other non-climate hazards could increase risks. Tables 2.5 and SM2.5 provide details of the key risks and temperature levels for the risk transitions.

<div id="FAQ 2.4" class="h2-container"></div>

<span id="faq-2.4-how-does-nature-benefit-human-health-and-well-being-and-how-does-climate-change-affect-this"></span>